1. Field of the Invention
The present invention relates to a packaging container used for a beverage or a food, in particular, to an aluminum-alloy sheet that is to be covered by a resin film and then formed into can bodies; and a method for producing the aluminum-alloy sheet.
2. Description of the Related Art
In drawn and ironed (DI) cans and bottle cans (hereafter, DI cans and bottle cans are collectively referred to as aluminum cans), the fabricability, flange formability, and curl formability (formability of opening portions of bottle cans) of the walls can be effectively improved by enhancing ductility of wall portions and neck portions after can forming and heat treatment and by making the Mn content in the aluminum alloy be less than a certain value (to reduce microprecipitates that strongly fix dislocations and inhibit formation of subgrains). On the other hand, Mn is considered as an essential element for achieving high ironing workability and high can strength. Typically, aluminum alloys have a Mn content of 0.5 mass % or more, often 0.8 mass % or more. Accordingly, improvements in the fabricability, flange formability, and curl formability have certain limits, which is a factor that hinders a decrease in the wall thickness and a decrease in the weight of aluminum cans.
Since aluminum-alloy sheets for producing aluminum cans desirably have high formability and a low earing ratio, complete recrystallization textures need to be formed in the stage of hot-rolled sheets. Accordingly, such aluminum-alloy sheets are generally produced in the following manners.
A double soaking in which a slab is subjected to a homogenization heat treatment at a high temperature of about 600° C., then cooled, and heated again is performed so that the solute Mn content of the slab is made less than a certain value and generation of microprecipitates is suppressed (precipitates are grown and made coarse); thus, the production conditions are controlled so that the complete recrystallization texture is obtained at the coiling temperature of the finish hot rolling. Other than the double soaking, there is also two-stage soaking in which a slab is subjected to a homogenization heat treatment at a high temperature of about 600° C., then cooled to about 500° C. at a certain cooling rate, and subsequently hot-rolled.
The techniques of producing aluminum-alloy sheets for aluminum cans from aluminum alloys having the above-described Mn content in the standard manners are disclosed in, for example, Patent Literatures 1 to 4 (Japanese Unexamined Patent Application Publication No. 2000-219929, paragraphs [0018] to [0020]; Japanese Unexamined Patent Application Publication No. 2007-204793, paragraph [0030]; Japanese Unexamined Patent Application Publication No. 2004-244701, paragraphs [0037] to [0038]; and Japanese Unexamined Patent Application Publication No. 2003-342657, paragraphs [0054] to [0062]).